The present invention relates to a method of producing, in large volumes and at low cost, titanium carbide, nitride or carbonitride whiskers generally submicron in diameter (having excellent reinforcing properties) suitable as reinforcement material in ceramic cutting tools, metals, intermetallics, plastics and metallic bonded hard materials.
During the last decades great progress has been made in the development of high-performance materials. One of the important reasons for this progress has been the production of inorganic (ceramic) whiskers which have high tensile strength and modulus values as well as resistance to high temperatures. The incorporation of these whiskers into advanced ceramics, metals and polymers has produced composites with superior properties.
Ceramic fibers can be produced by several methods and are most often in the form of single crystals or so-called "whiskers". Several types of ceramic whiskers (oxides, nitrides, carbides and borides) exist. For example, SiC-whiskers are well-known as a successful reinforcement in cutting tool materials. Most of the development work has hence been directed toward silicon carbide whiskers (e.g., Al.sub.2 O.sub.3 -matrix/SiC-whiskers composites). Some efforts have, however, also been made to develop other metal carbide, nitride and boride whiskers, particularly of transition metals such as Ti, Ta and Nb. Whiskers with diameters equal to or exceeding 1-2 micrometers are mostly obtained. Transition metal ceramic whiskers with submicron diameter are, however, not readily available which, so far, has made most of these whisker materials less interesting as reinforcement in different types of advanced ceramic materials.
Ceramic whiskers available commercially are in most cases very expensive and in the case of most transition metal whiskers, no commercial producer exists.
U.S. Pat. No. 3,754,076 discloses a method of producing silicon carbide whiskers by carbothermal reduction of rice hulls. Rice hulls contain both silica and a significant excess of carbon. When using rice hulls as a raw material, the rice hulls and silica therefore are intimately mixed. By heating the raw material, the cellulose give off gaseous products such as carbon dioxide, carbon monoxide, hydrogen, methane and water vapor. The product contains besides SiC whiskers and particles, a large amount of excess carbon which has to be removed.
In U.S. Pat. No. 4,248,844, SiC whiskers are produced in a similar process as in U.S. Pat. No. 3,754,076. A part of the rice hulls are, however, ashed in air to remove the carbon and then remixed with a carbonized fraction of rice hulls.
In U.S. Pat. No. 4,283,375, rice hulls are ashed in air to obtain amorphous SiO.sub.2 which is blended with fine particle sized carbon. This process is claimed to give .beta.-SiC whiskers with a good yield.
U.S. Pat. No. 4,284,612 describes a process of preparing SiC whiskers by using chopped carbon fibers made from partly oxidized organic fibers. The fibers should be approximately 10 to 100 .mu.m in length. The patent does not, however, give any explanation why the graphite fibers give a better whisker yield.
U.S. Pat. No. 5,221,526 uses silica and a particulate carbon source together with a boron component and a seeding component to control the shape and size of the produced SiC whiskers.
The carbothermal process of producing SiC whiskers has the general potential to produce large amounts to a low cost if the reaction condition can be properly controlled.
U.S. Pat. No. 4,888,084 discloses a method for the preparation of titanium nitride whiskers by carbothermal reaction of mixtures of TiO.sub.2, carbonized organic fibers and a catalyst such as Ni or Co. During the process, a halogen, preferably a chlorine-containing gas, is passed over the mixture. It is thought that the carbon fibers perform three functions: i) provide a high void volume necessary for the whisker growth; ii) act as a reducing agent for the titanium oxide; and iii) act as a nucleation site for the whisker growth.
In U.S. Pat. No. 5,256,243, a process of making TiC whiskers from a mixture of TiO.sub.2 or alkali metal compounds thereof, alkali metal chlorides and carbon by carbothermal reduction is described.
U.S. Pat. No. 4,883,559 discloses a process for making transition metal whiskers from melt of cyanide salt, alkali metal oxides and a transition metal oxide.
U.S. Pat. No. 5,094,711 discloses a process for producing single crystal titanium carbide whiskers by chemical vapor deposition (CVD).
U.S. Pat. No. 5,160,574 discloses a process for production of small diameter titanium carbide whiskers by a CVD method.
In the case of transition metal carbide, nitride or carbonitride whiskers, the carbothermal process has not been as successful as in the case of silicon carbide whisker synthesis. The CVD process works with a well-controlled composition of the gas phase, but the cost is high and the possibility of producing large amount of whiskers with submicron diameters is low.